The weight of vehicles is measured dynamically while they are moving in traffic on roadways with the aid of a method called Weigh-In-Motion (WIM).
Known WIM systems are based on “strip sensors”, which are arranged at intervals of a few meters in the longitudinal direction of the road. The flat, strip-like sensors are designed to function piezoelectrically and are embedded individually at a shallow depth in the road surface and secured in place. The width of the sensor in the direction of travel is usually in the order of a few centimeters, which is why the complete wheel contact surface cannot be evaluated. The piezoelectric effect means that detecting voltage signals is a very simple matter. Captured piezoelectric signals can be converted into force signals, and at the same time the cost of the strip sensors is extremely low. The strip sensors are connected via signal wires to an electronic signal detection unit, which can forward the measurement signals to an evaluation unit.
There are different layouts for the sensor arrangement, a full layout (FULL, FIG. 4a), according to US2011/0127090 for example, which is hereby incorporated herein in its entirety for all purposes by this reference, and a staggered layout (STAGGERED, FIG. 4b) according to WO02/23504.
As a vehicle drives over the sensor arrangement, measurement signals are captured with temporal resolution, from which it is possible to deduce the current vehicle speed. If vehicle wheels pass over two sensors with a given temporal difference, two measurement signals can be captured based on axle load, one from the first sensor and a later signal from the temporally offset sensor. The axle speed may be deduced from the distance between the two sensors, divided by the measured time difference between the signals from the two sensors, as shown in FIG. 4c. In order to calculate the axle loads from the sensor signals, the integrals of the sensor signals must be determined and multiplied with the vehicle speed and a calibration constant. This is necessary for WIM systems in which the sensors are thin strips, so that they are not contacted by the entire wheel surface.
For accurate weight measurements using strip sensors that do not permit direct weight detection, it is therefore imperative for the speeds to be measured extremely accurately.
However, the known sensor arrangements are not favourable for applications in which the speed of the vehicle varies as it travels over the WIM system, because the determination of weight becomes significantly less accurate. This is particularly true for applications in toll and control station areas, and other locations where precisely calibrated weight measurements are needed or desired, and the vehicles drive over the WIM systems slowly, at variable speeds, or even in “stop and go” tempo.